The present invention relates to a technical field of a negative pressure booster which is used in a brake booster or the like, and more particularly to a technical field of a negative pressure booster which is used as a brake booster or the like of a brake system in a vehicle which can obtain a deceleration during the usual braking operation of a vehicle having a large vehicle weight corresponding to a pedal stroke amount.
Conventionally, in a brake system of a vehicle such as an automobile, a negative pressure booster which makes use of negative pressure has been used in a brake booster. In such a conventional generally-used negative pressure booster, the interior of the negative pressure booster is partitioned into a constant pressure chamber into which negative pressure is introduced during the usual operation using a power piston and a variable pressure chamber in which the pressure varies. Further, during the usual braking operation with a usual step-in motion of a brake pedal, when an input shaft advances, a control valve is changed over and the atmosphere enters the variable pressure chamber. Then, the pressure difference arises between the variable pressure chamber and the constant pressure chamber and hence, the power piston is advanced. Accordingly, the negative pressure booster boosts an input of an input shaft attributed to a pedal step-in force at a given servo ratio and outputs the boosted force. Due to this output of the negative pressure booster, a master cylinder generates master cylinder pressure and a wheel cylinder is operated with the master cylinder pressure thus performing the usual braking operation.
Here, with respect to a vehicle such as a one box car or a recreational vehicle, recently, there has been observed a trend that a vehicle weight or a carrying weight is increased. Accordingly, in such a vehicle, along with the increase of the vehicle weight or the carrying weight, a brake manipulation quantity (a pedal stroke quantity) which becomes necessary during the usual braking operation is increased. In this manner, the brake manipulation quantity of a driver during the usual braking operation is increased and hence, it is difficult to achieve the favorable brake feeling.
On the other hand, in Japanese Patent Laid-open 2001-341632, there has been proposed a negative pressure booster which can obtain a large output even with a small pedal step-in force, that is, even with a small input and can perform a brake assist (hereinafter also referred to as “BA”) operation in case of emergency. According to the negative pressure booster disclosed in Japanese Patent Laid-open 2001-341632, even in a case that a moving speed of an input rod corresponding to the a pedal step-in speed is larger than a corresponding moving speed during the usual braking operation, when an input which is applied to the input rod is smaller than a given value, the BA operation is not performed, and only when the input which is applied to the input rod becomes equal to or above the given value, the BA operation is performed. That is, the negative pressure booster is configured to generate an output at the time of performing the BA operation which is larger than an output generated during the usual braking operation in response to the same input. In other words, at the time of performing the BA operation, the negative pressure booster can generate the larger output even with the input smaller than the input during the usual braking operation. Here, a stroke of an input rod of the negative pressure booster is shortened compared to a stroke when the large output can be obtained with the output equal to the output during the usual braking operation.
Further, in Japanese Patent Laid-open Heill (1999)-278245, there has been also proposed a technique which sets a servo ratio at a small value during the initial phase of the operation of the negative pressure booster and sets the servo ratio at a large value during the later phase of the operation of the negative pressure booster. In the negative pressure booster disclosed in the Japanese Patent Laid-open Heill (1999)-278245, a reaction mechanism includes a reaction disc, a spring and resilient means, wherein during the usual braking operation, a reaction is transmitted to a valve plunger by way of a reaction disc during the initial phase of the braking operation so as to decrease the servo ratio, and the reaction is transmitted to the valve plunger by way of the reaction disc and the spring during the later phase of the operation of the negative pressure booster thus increasing the servo ratio. Further, due to the provision of the reaction disc and the resilient means, hysteresis which exhibits the different outputs of the negative pressure booster between at the time of stepping in the brake pedal and at the time of releasing the step-in operation is obtained. Due to this hysteresis, the braking feeling is enhanced.
Here, it is considered that by applying the above-mentioned negative pressure booster disclosed in Japanese Laid-open Patent 2001-341632 to a vehicle which requires the deceleration higher than the deceleration during the usual braking operation, the large deceleration is obtainable with the small pedal step-in force. In this case, since the pedal stroke can be shortened, the brake feeling can be enhanced.
However, in the negative pressure booster disclosed in Japanese Patent Laid-open 2001-341632, the BA operation is performed only in the quick step-in operation in which the pedal step-in speed is faster than the step-in speed during the usual braking operation and, at the same time, the pedal stroke shortening function is performed. Accordingly, unless the pedal step-in speed is fast, the pedal stroke is not shortened and hence, it is difficult to obtain the favorable brake feeling. Further, there may arise a drawback that an operation sound is generated due to the engagement or the disengagement of an engaging portion of the BA mechanism.
Further, in the above-mentioned negative pressure booster disclosed in Japanese Patent Laid-open Heill(1999)-278245, a servo ratio is set at a small value during the initial phase of the braking operation within the usual braking operation region and the servo ratio is set at a large value in the later stage of the braking operation. Accordingly, in this negative pressure booster, no consideration is made with respect to the brake system of the vehicle which requires the deceleration higher than the deceleration during the usual braking operation. Further, in this negative pressure booster, although the brake feeling is enhanced due to the hysteresis between the step-in time of the brake pedal and the brake pedal release time, no consideration is made with respect to the deterioration of the brake feeling due to an increase of the pedal stroke at the time of performing the operation with the high deceleration.